Open Space Ratio

by | Dec 5, 2019 | NYC Zoning

Open Space Ratio is a Zoning term for the amount of open area on a zoning lot in the form of a percentage relative to the building square footage. A 10,000 square foot building with an Open Space Ration of 20% would have 2,000 square feet of open space.


Open Space Ratio


Open Space Definition

Open Space is any area on a zoning lot that is open to the sky from its lowest level. In certain circumstances open space may include covered areas.

An open space may include an area covered by a roof that is less than 10% of the total roof area if it is enclosed on at most one side or no more than 10% of the perimeter of the roofed area whichever is more.

Open space can be counted on the roof of the following:

  • Community Facility
  • Part of a residential building that does not have residences.
  • Non residential buildings if the building abuts a residential building. (see another post we wrote on the definition of abutting buildings)

Roofed Areas Counted for Open Space

Roofed areas must comply with the following to count as open space:

  • Maximum Height above curb level is 23 feet with some exceptions.
  • The roof must me at least 2.5 feet below the sill of any legally required window opening towards the roof area.
  • Must be accessible but not required in R8 or R9.
  • No dimension less than 25 feet. Except in R8 or R9 if it is on a rear yard of street line it may have a depth of 9 feet or a width at least twice its depth, or 50 feet, or the width of the lot whichever is less.


Open space Ratio Calculation

Open Space Ratio or OSR is the open area relative to the building floor area in the form of a percentage.

Open Space Ratio Calculation Example

We will do a sample calculation. If a building is 30,000 square feet and has an Open Space Ratio of %25 how many square feet would the lot require of open space?

Open Space Required = Building Area x OSR

Open Space Required = 30,000 x 25%

Open Space = 7,500 square feet

In this example the lot would need 7,500 square feet of open space.


Open Space Ratio is a critical aspect of Height Factor Zoning.

We have another post if you want to read about Height Factor Zoning.


Open Space Ratio Zoning

As an architect, I study Building and Zoning Codes closely, but these are complicated and quite involved issues. In this article, we reviewed some of the basic concepts with regards to the Open Space Ratio within NYC Zoning. This post does not assume to cover every possible issue or condition, but provide a general overview of the topic.


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Thank you for reading our blog post on Open Space Ratio NYC Zoning.

I hope this was helpful. Please leave questions and comments below. If you would like to speak with an architect you can Contact Fontan Architecture directly.


Jorge Fontan
Jorge Fontan

This post was written by Jorge Fontan AIA a Registered Architect and owner of New York City architecture firm Fontan Architecture. Jorge Fontan has earned 3 degrees in the study of architecture including two degrees from the City University of New York and a Masters Degree in Advanced Architectural Design from Columbia University. Jorge has a background in construction and has been practicing architecture for 15 years where he has designed renovations and new developments of various building types.